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Red seabream iridovirus (RSIV) is a major cause of marine fish mortality in Korea, with no effective vaccine available since its first occurrence in the 1990s. This study evaluated the efficacy of a formalin-killed vaccine against RSIV in rock bream under laboratory and field conditions. For the field trial, a total of 103,200 rock bream from two commercial marine cage-cultured farms in Southern Korea were vaccinated. Farm A vaccinated 31,100 fish in July 2020 and monitored them for 18 weeks, while farm B vaccinated 30,700 fish in August 2020 and monitored them for 12 weeks. At farm A, where there was no RSIV infection, the vaccine efficacy was assessed in the lab, showing a relative percentage of survival (RPS) ranging from 40% to 80%. At farm B, where natural RSIV infections occurred, cumulative mortality rates were 36.43% in the vaccinated group and 80.32% in the control group, resulting in an RPS of 54.67%. The RSIV-infectious status and neutralizing antibody titers in serum mirrored the cumulative mortality results. This study demonstrates that the formalin-killed vaccine effectively prevents RSIV in cage-cultured rock bream under both laboratory and field conditions. [ABSTRACT FROM AUTHOR]

Abstract: MVSF‐12 belonging to megalocytivirus type 3 was isolated from cultured starry flounder; Platichthys stellatus, at the moribund or subclinical stage with low mortalities in Korea. Of 20 apparently healthy fish in the farms, 17 were also confirmed in nested polymerase chain reaction to be infected by megalocytivirus. When starry flounder; rock bream, Oplegnathus fasciatus; and olive flounder, Paralichthys olivaceus, were artificially infected by MVSF‐12 or iridovirus sachun‐1 (IVS‐1, megalocytivirus type 1), starry flounder and olive flounder showed no mortality until Day 24, without any clinical signs including enlarged spleen, while rock bream showed 100% mortality by IVS‐1 infection within 11 d but no mortality by MVSF‐12. Although it was not pathogenic, MVSF‐12 in infected fish at Day 24 was viable when successfully cultured in vitro using primary rock bream embryo cells and produced a cytopathic effect (CPE) with the viral copy numbers between 1.76 × 107 and 5.23 × 107/mL of culture supernatant. In conclusion, this study demonstrates the low pathogenicity of MVSF‐12 and low susceptibility of starry flounder and olive flounder to both MVSF‐12 and IVS‐1. Indeed, MVSF‐12 at the subclinical stage could be replicated with CPE in vitro, indicating a possibility to induce pathogenic effects and mortality under adverse environment or physiologic conditions. [ABSTRACT FROM AUTHOR]

This study has assessed the effects of vitamin E (−E, +LE, +HE; 0, 100, 1000 mg/kg, respectively) in fish diets containing high levels ( HL; 10%) of fish oil ( FO) or mixed vegetable oils ( VO) on the growth and inflammatory, oxidative stress, and apoptotic gene expression in the head kidney of olive flounder, Paralichthys olivaceus. Consequently, the highest weight gain was achieved in the FO group and the lowest in the HL-VO + LE group. The gene expression levels of each group were compared to the 5% FO group. The 5% VO group showed higher expression levels of tumor necrosis factor ( TNF) α, interleukin (IL)-1β, and scinderin-like ( ScinL) genes. Although lysozyme gene expression was higher in the HL-VO + LE group, the other gene expression levels of the HL-FO/ VO + LE groups were not different from those of the FO group. The HL-FO/ VO−E/+ HE groups showed a higher TNFα gene expression, but the cytochrome oxidase subunit III gene expression was higher in the HL-FO−E and HL-VO + HE groups. Lysozyme gene expression was higher in the HL-FO−E and HL-VO−E/+ HE groups. IL-6 and ScinL gene expression were higher in the HL-VO-E and HL-VO + HE groups, respectively. In conclusion, mixed VO and too high or too low vitamin E levels in fish diets may affect inflammatory, oxidative stress, and apoptotic gene expression in the head kidney of olive flounder. [ABSTRACT FROM AUTHOR]

Since bacterial infection cause a significant economic loss in fish farms, it is necessary to develop rapid diagnostic tools. Interests on label-free biosensors have been raised for the rapid detection of aquatic pathogenic bacteria but have not been extensively studied yet. Here we report a quartz crystal microbalance ( QCM) immunosensor system for the rapid and simple detection of Aeromonas hydrophila, a pathogen for fish and human, in comparison with a conventional indirect ELISA method. In QCM immunosensor system, an antibody against A. hydrophila was covalently cross-linked to the gold surface of sensor chip and bacterial attachment was monitored as real-time frequency shifts within 5 min. The frequency shifts were very positively related to the amounts of bacterial cells between 6.25 and 100 μg corresponding to 6 × 106 to 108 CFU with a high specificity. The QCM immunosensor was also able to detect bacterial cells in fish tissue extract in a dose-dependent manner. Indirect ELISA also showed the dose-dependent reaction and the amplified signal may allow a lower detection limit. However, QCM immunosensor system showed a more linear and reliable standard curve with R2 value of almost 1 (0.9999). Moreover, detection of the bacteria was much quicker and simpler. [ABSTRACT FROM AUTHOR]